Autosomal Dominant Polycystic Kidney Disease is the most common form of inherited kidney failure world- wide and is caused by mutations in two genes, PKD1 and PKD2. Of the many extrarenal manifestations associated with ADPKD, vascular complications such as aneurysms remain the most devastating and can result in substantial morbidity and mortality. Although there are no adult mouse models that can be reliably used to study this phenotype, mice with germ line mutations in Pkd1 or Pkd2 die in mid-gestation due to a vascular phenotype characterized by gross edema and hemorrhage. We have been using these models to understand the role of Pkd1 and Pkd2 in endothelial cells. We have made the novel discovery that edema in Pkd mutant mice is due to abnormal lymphatic morphogenesis with associated defects in oriented cell migration. Preliminary data developed over the last funding cycle may link polycystin deficiency in lymphatic endothelial cells (LECs) to altered fatty acid metabolism which has been shown to result in disruption of the lymphangiogenic program. Mice with LEC deletion of Cpt1a, a fatty acid transport protein, have edema, abnormal lymphangiogenesis and a phenotype that closely resembles that of Pkd1/2 null embryos. In addition, we have shown that mice with a knock-in mutation that abrogates PC1 cleavage have normal vascular and lymphatic development. Since Polycystin-1 cleavage is necessary for ciliary trafficking, this indicates that a subset of polycystin signaling pathways may not originate from the cilium. We have developed a series of aims that will expand on these preliminary observations. In Aim 1 we will use a combination of methods to test whether loss of Pkd1/2 phenocopies the loss of Cpt1a on a molecular level. In Aim 2, we will test whether the primary function of full-length, uncleaved polycystin-1 is extra-ciliary in lymphatic endothelial cells and whether cilia are required for lymphatic development in mice. Aim 3 will further explore the role of polycsytins in non- lymphatic endothelial cells using in vivo and in vitro model systems. In this proposal we will build on novel preliminary observations and delve into the cellular function of polycystins using endothelial cells as a model system. We expect that this work will provide fundamental insights into the role that polycystins play in lymphangiogenesis and vascular development. We anticipate that the paradigms that we develop will have broad implications for defining key aspects of polycystin biology.